영문제목Kinetics and Mechanism of the Aminolysis of Phenacyl Bromides in Acetonitrile. A Stepwise Mechanism with Bridged Transition State

초록

In the aminolysis of phenacyl bromides (YC6H4COCH2Br) with benzylamines (XC6H4CH2NH2) in acetonitrile, the Bronsted bX ( bnuc) values observed are rather low ( bX = 0.69-0.73). These values are similar to those ( bX @ 0.7) for other aminolysis reactions of phenacyl compounds with anilines and pyridines, but are much smaller than those ( bX = 1.1-2.5) for the aminolysis of esters with benzylamines which are believed to proceed stepwise with rate-limiting expulsion of the leaving group. The relative constancy of the bX values ( bX @ 0.7) irrespective of the amine, leaving group and solvent can be accounted for by a bridged type transition state in the rate-limiting expulsion of the leaving group. Thus the aminolysis of phenacyl derivatives are proposed to proceed stepwise through a zwitterionic tetrahedral intermediate (T±), with rate-limiting expulsion of the leaving group from T±. In the transition state, the amine is bridged between the carbonyl and a-carbons, which leads to negligible effect of amine on the leaving group expulsion rate.